The present embodiments relate to a breast coil for a magnetic resonance tomography device.
With the increasing spread of magnetic resonance tomography, more and more breast investigations are being carried out with the help of this imaging method. One advantage lies in the fact that, unlike the customary mammographic methods, magnetic resonance tomography does not subject the breast to X-ray doses. In a magnetic resonance tomography device, the body or body region that is to be investigated is customarily exposed to a defined magnetic field (e.g., the B0 field) with the help of a basic magnetic field system. In addition, a magnetic field gradient is applied with the help of a gradient system. Via a high-frequency transmission signal and using suitable antennas, high-frequency magnetic resonance excitation signals (HF signals) are emitted with a defined field strength. The emission of the high-frequency signals for the purpose of nuclear spin magnetization may be effected using a “whole-body coil” or “body coil” (e.g., a birdcage antenna) permanently built into the magnetic resonance tomograph. The whole-body coil surrounds the measurement space (e.g., the “patient tunnel”), in which the female patient is positioned during the investigation. The magnetic flux density of this excitation field may be referred to as B1 and, accordingly, the pulsed high-frequency field is also referred to as the B1 field. Using these HF pulses, the nuclear spin of certain atoms, which are resonantly excited by the B1 field, is tilted by a defined flip angle relative to the magnetic field lines of the B0 field. Relaxation of the nuclear spin is accompanied by the re-emission of high frequency signals (e.g., magnetic resonance signals). The magnetic resonance signals may be received using the whole body coil. For this purpose, local coils that have a higher signal/noise ratio may be used. These are antenna systems affixed in the immediate neighborhood of the patient. The magnetic resonance signals induce a voltage in the individual antennas of the local coils. The induced voltage is then amplified using a low-noise preamplifier (LNA, preamp) and is forwarded to the receiving electronics. Over the course of time, special local coils have been made available for the most varied of application situations (e.g., head coils for investigations in the region of the head or the breast coils mentioned for investigations on female breasts). Such a breast coil may have a coil housing (e.g., with two breast recesses that have a roughly circular cross-section and are arranged beside each other, in which the breasts are accommodated). Arranged in the housing and around the breasts there may, for example, be several coil elements in the form of conducting loops with appropriate electronic components.
From the magnetic resonance signals that are acquired or the “raw data”, the desired magnetic resonance image data (MR image data) may be reconstructed. Location encoding is effected by the switching of appropriate magnetic field gradients in the various spatial directions at precisely defined times (e.g., during the emission of the HF signals and/or on receipt of the magnetic resonance signals). Each image point in the magnetic resonance image is, for example, assigned to a small spatial volume (e.g., a so-called voxel), and each brightness or intensity value of the image points is linked to the signal amplitude of the magnetic resonance signal received from this voxel. The relationship between an HF pulse resonantly radiated-in with a field strength B1 and the flip angle α produced by this is given by the equation
                    α        =                              ∫                          t              =              0                        τ                    ⁢                      γ            ·                                          B                1                            ⁡                              (                t                )                                      ·                                                  ⁢                          ⅆ              t                                                          (        1        )            where γ is the gyromagnetic ratio that, for most nuclear spin investigations, may be regarded as a fixed material constant. τ is the duration of application of the high frequency pulse. The flip angle produced by an emitted HF pulse, and hence the strength of the magnetic resonance signal, consequently depends on the duration of the HF pulse and the strength of the irradiated B1 field. Spatial fluctuations in the field strength of the excitation B1 field thus lead to unwanted variations in the magnetic resonance signal received, which may falsify the resulting measurement.
An unfavorable feature is that it is at high magnetic field strengths (e.g., inevitably present in a magnetic resonance tomograph due to the basic magnetic field B0 required) that inhomogeneous penetration characteristics are exhibited in conductive and dielectric media such as, for example, tissues. A result of this is that the B1 field within the measured volume may vary greatly. In the case of the high Larmor frequencies, used for ultra-high field magnetic resonance investigations, the conductivity of the tissue is high. This leads to especially large inhomogeneities, so that in the case of magnetic resonance measurements with a basic magnetic field of three Tesla or more, special measures are to be taken in order that an adequately homogeneous distribution of the HF field transmitted by the high-frequency antenna is achieved through the entire volume. In the case of breast imaging, eddy currents arise in the patient's body (e.g., in the region of the thorax or abdomen) or in the individual breasts with the high basic field strengths due to the B1 field. These lead to an asymmetric distortion of the B1 field (e.g., of the transmitted excitation field) and often to an overshadowing of deeper-lying tissues. In the breasts, this produces both a B1 field pattern with a left/right oriented asymmetry and also a substantial asymmetry in the contrast for the left breast compared to the right breast. This unwanted inhomogeneity has a detrimental effect on the diagnostic relevance or predictive capacity of the magnetic resonance images produced.
In order to reduce the distortion of the B1 field by the patient's body, the body coil is driven such that the body coil transmits with a permanently-set elliptical polarization in order to counteract the asymmetry of the patient's body caused by the greater elongation in the right/left direction than in the anterior/posterior direction. Alternatively, for the purpose of adapting the B1 field, a system of transmission antennas, in which two or more are used as two independent channels, is used so that the amplitude relationship and phase relationship between these channels may be individually adjusted. However, such transmission systems are relatively expensive. In addition, this does not solve the problems for magnetic resonance tomography devices that already exist.